3. Who is this man? I ... (to think) that I ... (to know) him, but I ... (to forget) his name.

3. Who is this man? I think that I know him, but I forget his name.

В третьем предложении нужно использовать Present Simple для выражения настоящего состояния или мнения. Форма глагола "to think" в Present Simple - think, форма глагола "to know" - know, а форма глагола "to forget" - forget.

Ответ: Who is this man? I think that I know him, but I forget his name.